Input buffers are used for a wide variety of functions in integrated circuits. Buffers generally have a high input impedance to avoid excessively loading circuits to which they are connected, and, conversely, have a low output impedance to drive electrical circuits without excessive loading. Buffers are typically used in digital circuits to condition electrical signals applied to internal circuitry so that internal signals are generated with well-defined logic levels and transition characteristics. For example, buffers may be utilized for coupling command, address and write data signals from respective buses in a memory device, such as a dynamic random access memory (“DRAM”) and a synchronous dynamic random access memory (“SDRAM”), so that clean, unambiguous signals are properly received by various components of the memory device.
Input buffer circuits may be used to convert high speed, small swing input signals to digital signals, such as signals required by internal circuitry in memory devices. Differential input buffers conventionally include differential amplifiers, which are symmetrically structured and typically have a differential pair of input terminals and/or output terminals. The symmetrical topography of these differential amplifiers causes them to operate in a symmetrical manner when they receive complementary signals. Differential input buffers are particularly useful in digital circuits for determining whether a single input signal is above a fixed reference voltage, signifying a logic “1” or below the fixed reference voltage, signifying a logic “0”. However, in such cases, the input buffers receive a single input signal rather than two complementary input signals. This lack of symmetry in applying signals to the input buffers can cause them to operate in a non-symmetrical manner. As a result, they may not respond to an input signal transitioning from a first level to a second level in the same manner that they respond to an input signal transitioning from the second level to the first level. Moreover, input buffers respond faster to a differential input and hence, can be used at higher frequencies for differential inputs.
There is, therefore, a need for an input buffer that operates more symmetrically when receiving a single-ended input signal so that it responds to transitions of the input signal in one direction in the same manner that it responds to transitions of the input signal in the opposite direction.